Datasheet LT6600-10 (Analog Devices) - 8

LT6600-10
APPLICATIONS INFORMATION Interfacing to the LT6600-10
Use Figure 4 to determine the interface between the LT6600-10 and a current output DAC. The gain, or Note: The referenced pin numbers correspond to the S8 “transimpedance”, is defi ned as A = V package. See the Pin Functions section for the equivalent OUT/IIN Ω. To compute the transimpedance, use the following equation: DFN-12 package pin numbers. The LT6600-10 requires 2 equal external resistors, RIN, to A = 402 •R1Ω set the differential gain to 402Ω/R R1+ R2 IN. The inputs to the fi lter are the voltages V + – IN and VIN presented to these external By setting R1 + R2 = 402Ω, the gain equation reduces components, Figure 1. The difference between V + IN and to A = R1Ω. V – + IN is the differential input voltage. The average of VIN and V – IN is the common mode input voltage. Similarly, The voltage at the pins of the DAC is determined by R1, the voltages V + – + OUT and VOUT appearing at Pins 4 and 5 R2, the voltage on VMID and the DAC output current (IIN of the LT6600-10 are the fi lter outputs. The difference or I – IN ). Consider Figure 4 with R1 = 49.9Ω and R2 = between V + – OUT and VOUT is the differential output voltage. 348Ω. The voltage at VMID is 1.65V. The voltage at the The average of V + – OUT and VOUT is the common mode DAC pins is given by: output voltage. R1 R1• R2 V = V • +I Figure 1 illustrates the LT6600-10 operating with a single DAC PIN7 R1+ R2 + 402 IN R1+ R2 3.3V supply and unity passband gain; the input signal is = 103mV +I 43.6Ω IN DC coupled. The common mode input voltage is 0.5V and the differential input voltage is 2V – + P-P. The common mode IIN is IIN or IIN .The transimpedance in this example is output voltage is 1.65V and the differential output voltage 50.4Ω. is 2VP-P for frequencies below 10MHz. The common mode output voltage is determined by the voltage at VOCM. Since
Evaluating the LT6600-10
VOCM is shorted to VMID the output common mode is the The low impedance levels and high frequency operation mid-supply voltage. In addition, the common mode input of the LT6600-10 require some attention to the matching voltage can be equal to the mid-supply voltage of VMID (refer networks between the LT6600-10 and other devices. The to the Distortion vs Input Common Mode Level graphs in previous examples assume an ideal (0Ω) source impedance the Typical Performance Characteristics section). and a large (1kΩ) load resistance. Among practical examples Figure 2 shows how to AC couple signals into the where impedance must be considered is the evaluation LT6600-10. In this instance, the input is a single-ended of the LT6600-10 with a network analyzer. Figure 5 signal. AC-coupling allows the processing of single-ended is a laboratory setup that can be used to characterize the or differential signals with arbitrary common mode levels. LT6600-10 using single-ended instruments with 50Ω The 0.1μF coupling capacitor and the 402Ω gain setting source impedance and 50Ω input impedance. For a unity resistor form a high pass fi lter, attenuating signals below gain confi guration the LT6600-10 requires a 402Ω source 4kHz. Larger values of coupling capacitors will proportion- resistance yet the network analyzer output is calibrated ally reduce this highpass 3dB frequency. for a 50Ω load resistance. The 1:1 transformer, 53.6Ω and 388Ω resistors satisfy the two constraints above. In Figure 3 the LT6600-10 is providing 12dB of gain. The The transformer converts the single-ended source into a gain resistor has an optional 62pF in parallel to improve differential stimulus. Similarly, the output the LT6600-10 the passband fl atness near 10MHz. The common mode will have lower distortion with larger load resistance yet output voltage is set to 2V. the analyzer input is typically 50Ω. The 4:1 turns (16:1 impedance) transformer and the two 402Ω resistors of 66001fe 8